1. Field of the Invention
The present invention relates to a communication system, and more particularly to a method and a system for allocating downlink resources in a communication system.
2. Description of the Related Art
In the next generation communication systems, active research is in progress to provide users with high-speed services having various Qualities of Service (QoS). Especially in the current next generation communication systems, active research is in progress to guarantee mobility and QoS for Broadband Wireless Access (BWA) communication systems, such as a Wireless Local Area Network (WLAN) system and a Wireless Metropolitan Area Network (WMAN) system. Representatives of such communication systems include an Institute of Electrical and Electronics Engineers (IEEE) 802.16a/d communication system and an IEEE 802.16e communication system.
The IEEE 802.16a/d communication system and the IEEE 802.16e communication system, which are the BWA communication systems, employ an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme in order to support a broadband transmission network for a physical channel of the WMAN system. The IEEE 802.16a/d communication system considers only a single cell structure with a state in which a current Subscriber Station (SS) is stationary, without considering mobility of the SS at all. However, the IEEE 802.16e communication system considers mobility of the SS in addition to the IEEE 802.16a communication system. As used herein, an SS having mobility is called a Mobile Station (MS).
The IEEE 802.16e communication system transmits a plurality of sub-carriers while maintaining the orthogonality between them, thereby achieving an optimum transmission efficiency for high-speed data transmission. Further, the IEEE 802.16e communication system has a good frequency use efficiency and is robust against multi-path fading, so that the communication system can achieve an optimum transmission efficiency for high-speed data transmission. Further, because the OFDM/OFDMA scheme uses overlapping frequency spectrums, the communication system effectively uses the frequency and is robust against the frequency selective fading and the multi-path fading. Moreover, the OFDM/OFDMA scheme can reduce the influence of an Inter-Symbol Interference (ISI) by using a guard interval, and can achieve simple design in a hardware structure of an equalizer.
Meanwhile, the OFDM/OFDMA scheme properly distributes resources in order to enhance the degree of use of channels between a Base Station (BS) and a plurality of MSs located within one cell. In a communication system employing the OFDM/OFDMA scheme, one of sharable resources is a sub-carrier, and an optimum degree of channel use is guaranteed according to how sub-carriers are allocated to MSs within a cell. Here, a set including at least one sub-carrier is a sub-channel.
Further, in the IEEE 802.16e communication system, data is transmitted frame by frame, and each frame is divided into an interval for transmitting downlink data and an interval for transmitting uplink data. An uplink data interval and a downlink data interval are divided according to a frequency axis and a time axis. Each element two-dimensionally arranged according to the frequency axis and the time axis is called a slot. Therefore, for allocation of downlink data bursts of an MS, the BS uses a normal MAP, new a normal MAP, or a Hybrid Automatic Repeat reQuest (H-ARQ) MAP. The data bursts occupy multiple time slots and are allocated to the downlink data interval.
However, there has been no specific scheme for allocating the data bursts to data burst areas of a frame in the IEEE 802.16e communication system. Therefore, the IEEE 802.16e communication system requires a scheme for allocation and operation of data burst regions in the frame. Further, in the IEEE 802.16e communication system, which is a BWA communication system, a BS efficiently allocates resources of each frame to MSs and transmits the resource allocation information to the MSs through a MAP message. Here, a MAP message transmitting downlink resource allocation information is a DownLink MAP (DL-MAP) message and a MAP message transmitting uplink resource allocation information is an UpLink MAP (UL-MAP) message.
When the BS has transmitted the downlink resource allocation information and the uplink resource allocation information through the DL-MAP message and the UL-MAP message, each of the MSs can detect locations of the resources allocated to the MS itself and control information of data to be received by the MS itself, by decoding the DL-MAP message and the UL-MAP message. By detecting the allocated resource locations and the control information, the MSs can receive or transmit data through the downlink and uplink.
Meanwhile, the MAP message includes different MAP Information Element (IE) formats according to the type of link (downlink or uplink) and according to the type of data bursts (according to whether the data bursts employ the HARQ scheme, do not employ the HARQ scheme, or include control information). Therefore, the MSs can decode each MAP IE only when they are aware of each MAP IE format in advance. Each MAP IE can be identified by using a Downlink Interval Usage Code (DIUC) in the case of the downlink, while it can be identified by using an Uplink Interval Usage Code (UIUC) in the case of the uplink.
In the IEEE 802.16e communication system, data is transmitted to a plurality of MSs located within one cell during one frame, and the resources allocated to each MS located within the cell for data transmission have changeable sizes. Therefore, when data is transmitted to a plurality of MSs located within one cell in the IEEE 802.16e communication system, a DL-MAP message indicating allocation information of resources allocated to each MS for data transmission is transmitted by the same frame as that of the data to be transmitted. Further, as the size of the DL-MAP message increases, the resources to be allocated for the data to be transmitted decreases. As a result, the resources to be allocated for data transmission may become diminished.